PALEOZOIC ERA 

 CHAPTER VI 



THE CAMBRIAN PERIOD 



The Cambrian represents the earliest period of the great Pal- 

 eozoic era, and the rocks which make up the Cambrian system 

 include the oldest known of the normal fossiliferous strata. Since 

 these strata are the oldest which carry abundant organic remains, 

 it follows that they are the earliest formed rocks to which the ordi- 

 nary methods of subdividing and correlating rock masses can be 

 applied. From the Cambrian on, the legible records of events of 

 earth history are far more abundant and less defaced than those of 

 pre-Cambrian time. From now on we shall be able to trace the 

 changing outlines of the relief features of the continents and the 

 evolution of organisms with some degree of definiteness and satis- 

 faction, though a vast amount of work yet remains to be done both 

 as regards discovery of new records and the interpretation of 

 records old and new. 



Origin of Name, Subdivisions, Etc. 



The oldest Paleozoic rocks were first carefully studied independ- 

 ently in the British Isles by the two able geologists, Sedgwick and 

 Murchison, before the middle of the nineteenth century. Murchi- 

 son applied the name " Silurian" to the great series of oldest 

 fossiliferous strata and divided them into Lower and Upper 

 Silurian. Sedgwick, however, considered that the very oldest 

 fossil-bearing rocks should be separately designated, hence his 

 application of the term " Cambrian, " from Cambria an old Latin 

 name for a part of Wales. The Cambrian is now recognized the 

 world over as the oldest Paleozoic system. 



In North America a threefold subdivision of the Cambrian 

 system is recognized as follows: 



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